1. Field of the Invention
This invention relates to a control unit and a control method of a robot, which make it possible to objectively decide whether or not the arc welding executed by a robot is normally performed using output data obtained in the welding and data predetermined for executing the welding.
2. Description of the Related Art
The quality control of products worked with arc welding using a robot has been performed by an operator or a person in charge or inspection who visually checks how weld beads are formed or melted on all machined or sampled work-pieces. Because this type of evaluation depending on the human subjective decision requires skill and tends to bring about personal errors, it lacks in objectively as the data for quality control.
Moreover, it is conceivable for the time being to check the quality of welded workpieces by non-destructive inspections such as X-ray fluoroscopy and ultrasonic flow detection. However, large systems and a lot of time are necessary to execute these methods and moreover considerable cost and time are consumed since the inspection is executed for every workpiece every time welding is finished.
Furthermore, since these existing methods have been used to evaluate whether or not welding is normally performed, after the welding is completed, these methods have not been applied to decide whether or not welding is normally performed, during the welding.
As explained before, whether or not welding is normally performed has not been objectively evaluated during the welding by the existing methods, hence it has been difficult for an operator to take a quick action when a defective weld occurs.
Moreover, in the existing methods, various data such as a value outputted from a robot control unit to an arc welding portion and a feedback value inputted from the arc welding portion to the robot control unit have not been cumulatively recorded when a defective weld is detected in accordance with predetermined objective criteria. Therefore, it has been impossible to obtain data to be used as a guideline for improvement or data for assuring the welding quality.
It is an object of the present invention to provide a robot control unit and a robot control method, in which an index enabling to decide whether there is a possibility of the occurrence of defective weld is calculated in accordance with data such as a command value outputted from a control unit of a robot used for arc welding to an arc welding portion (welding power supply portion) and a data such as a feedback value inputted from the arc welding portion to the control unit when welding is executed under control of the control unit, and the welding state is cumulatively recorded in a memory when it is decided according to the index that there is a possibility of the occurrence of defective weld.
The present invention makes it possible to immediately give an alarm to arouse operator""s attention when it is decided that there is a possibility of the occurrence of defective weld, and moreover makes it possible to create defective weld history data by cumulatively storing the state of welding being performed at the point of time in a memory in succession.
The present invention is summarized below.
While operating a robot by reproducing a robot operation program, a welding voltage command value Vc and a welding current command value Ic, both of which serve as command values outputted from a robot control unit to an arc welding portion, and moreover, an actual welding voltage Vf and an actual welding current If, both of which serve as feedback values outputted from the arc welding portion to the robot control unit, are monitored respectively.
Then, two types of indexes (hereinafter referred to as a relative voltage deviation and a relative current deviation respectively are calculated in accordance with the above data as shown below.
Relative voltage deviation xcex94Vr is given as:
xcex94Vr=|Vcxe2x88x92Vf|/Vc
Relative current deviation xcex94Ir is given as:
xcex94Ir=|Icxe2x88x92If|/Ic 
It is preferable to use both the above indexes to make it possible to decide whether or not there is a possibility of the occurrence of defective weld. However, it may also be decided by using only either of the above indexes.
Criteria by which an occurrence of defective weld can be decided from the calculated indexes are set to the robot control unit. The criteria are set to the control unit generally in the form of a threshold value xcex94Vrth with respect to relative voltage deviation xcex94Vr and a threshold value xcex94Irth with respect to relative current deviation xcex94Ir.
That is, the threshold values xcex94Vrth and xcex94Irth are previously set to the control unit, and the relative voltage deviation xcex94Vr and the relative current deviation xcex94Ir are always obtained while welding is executed. Then, it is decided that the welding is not normally performed when either of the above deviations exceeds the threshold value xcex94Vrth or xcex94Irth. Moreover, instead of the above way, it is possible to decide whether or not the welding is normally performed by adopting, for example, an orthogonal coordinate system in which xcex94Vr is assigned to x axis and xcex94Ir is assigned to y axis. In this case, when one point (xcex94Vr,xcex94Ir) showing relative voltage deviation xcex94Vr and relative current deviation xcex94Ir at each point of time is plotted in the coordinate, it is judged whether the plotted point is included in a normal area or a defective area which has been divided in advance in the coordinate.
In case of these various criteria, particularly, in the former case, it is preferable that a user can select a mode for successively obtaining the relative voltage deviation xcex94Vr and the relative current deviation xcex94Ir and monitoring whether either of the deviations exceeds its corresponding threshold value or a mode for monitoring whether only one of the deviations exceeds its corresponding threshold value without monitoring other indexes. Further, it is preferable that a user can set threshold values (values xcex94Vrth and xcex94Irth) which are appropriate by selecting them each time taking account of the operating conditions. Moreover, after executing several statements of blocks (lines) constituting an operation program, it is possible to change the criteria (mode and threshold value) to other criteria for the next block forward.
When it is decided in accordance with the criteria set to the control unit of a robot that the welding now under execution is not normally performed (that is, that a defective weld occurs), the relevant data showing the operation state at the point of time is stored as defective weld history data. The relevant data include the following information.
(a) Data showing the name and line number of a program being executed when a defective weld occurs
(b) Data showing the command value and actual value of welding voltage and the command value and actual value of welding current
(c) Data for the date and time when a defective weld occurs
(d) Data for specifying the work with a defective weld
(e) Data showing the cumulative defective weld occurrence frequency
It is practical to store the data showing the defective weld occurrence frequency by adding 1 to a register whenever a possibility of the occurrence of defective weld is detected. For notification of a defective weld, it is preferable to use a system for generating a predetermined digital signal when the defective weld occurs and inputting the signal to a register. It is possible to activate warning means such as a buzzer or a red lamp by using the values stored in the register. It is also possible to adopt a system in which a user can specify a register area to be assigned in the register with register numbers.